EVDO provides for the wireless transmission of data using radio signals and is typically, deployed alongside a wireless carrier's voice services. QoS (quality of service) capabilities were first introduced to EVDO in a revision of the standard referred to as the IS-856-A Protocol Standard. Later revisions also support QoS capabilities. If voice traffic is carried over EVDO, it is typically carried using VoIP (voice over IP), and a QoS mechanism is provided to ensure delay sensitivity requirements for VoIP are satisfied. QoS flows for VoIP include a signalling flow and a media flow having QoS parameters suited for VoIP. Another flow may be set up for other traffic such as e-mail, internet etc. The other flow may, for example, be serviced on a best effort basis. The best effort flow is the default flow where all other data traffic goes through, not part of QoS.
An EVDO subnet is a group of cells that share the same identifier and within which a mobile device can move around without losing its initial session configuration. When a mobile device moves from a current serving EVDO subnet to a new EVDO subnet, the device assumes QoS flows will be transferred properly. When the mobile device attempts to make a new VOIP call in the new subnet, it is assumed that the QoS flows are already in place, and as such low latency performance is expected.